A display device using a liquid crystal compound (in this application, the term “a liquid crystal compound” is used as a generic term for a compound having a liquid crystal phase and a compound having no liquid crystal phases but useful as a component of a liquid crystal composition) has been widely used for displays of watches, calculators, word processors and so forth. In this display device, the refractive index anisotropy, the dielectric anisotropy and so forth of the liquid crystal compound are utilized.
In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes modes of phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA) and polymer sustained alignment (PSA). A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal-insulator-metal (MIM) and so forth.
These liquid crystal display devices contain a liquid crystal composition with suitable physical properties. It is desirable that the liquid crystal composition has suitable physical properties in order to improve the characteristics of the liquid crystal display device. General physical properties necessary for the liquid crystal compound that is a component of the liquid crystal composition are as follows.
(1) chemically stable and physically stable,
(2) a high clearing point (transition temperature between a liquid crystal phase and an isotropic phase),
(3) a low minimum temperature of a liquid crystal phase (a nematic phase, a smectic phase and so forth), especially a low minimum temperature of the nematic phase,
(4) an excellent compatibility with other liquid crystal compounds,
(5) a large dielectric anisotropy, and
(6) a large refractive index anisotropy.
A voltage holding ratio can be increased when a composition including a chemically and physically stable liquid crystal compound, as described in item (1), is used for a display device. The temperature range of a nematic phase can be increased in a composition that includes a liquid crystal compound having a high clearing point or a low minimum temperature of a liquid crystal phase as described in items (2) and (3), and thus the device can be used in a wide temperature range.
A liquid crystal compound is generally used in the form of a composition prepared by mixing it with many other liquid crystal compounds in order to exhibit characteristics that are difficult to be attained by a single compound. Accordingly, it is desirable that a liquid crystal compound used for a display device has an excellent compatibility with other liquid crystal compounds and so forth, as described in item (4).
Recently, a liquid crystal display device having a better quality especially of display performance such as characteristics of contrast, display capacity and response time is required. Moreover, a liquid crystal material suitable for a low driving voltage is required, that is to say, a liquid crystal compound that makes it possible to decrease the threshold voltage, and a liquid crystal composition that includes the compound and is suitable for a low driving voltage are required.
Threshold voltage (Vth) is expressed, as is well known, by the following equation; See H. J. Deuling, et al., Mol. Cryst. Liq. Cryst., 27 (1975) 81:Vth=π(K/∈0Δ∈)1/2 where K is an elastic constant, ∈o is a dielectric constant in a vacuum. As the equation shows, there are two possible ways in order to decrease Vth: the value of Δ∈ (dielectric anisotropy) is increased or the value of K is decreased. Since it is still hard to control the value of K by use of the present technology, such demand is dealt with the use of a liquid crystal material having a large Δ∈ at present stage. Under the circumstances, a liquid crystal compound having a large dielectric anisotropy, as described in item (5), has been studied eagerly.
It is desirable for an excellent liquid crystal display that the value of the thickness of the liquid crystal display device and the refractive index anisotropy (Δn) of the liquid crystal material used is constant. See E. Jakeman, et al., Phys. Lett., 39A. 69 (1972). The response speed of the liquid crystal display device is inversely proportional to the square of the cell thickness. Then, a liquid crystal composition having large refractive index anisotropy should be used in order to produce a liquid crystal display device that is able to respond at high speed and thus can be applied to the display of moving images and so forth. Accordingly, a liquid crystal compound having a large refractive index anisotropy as is described in item (6) is desired.
A variety of liquid crystal compounds having a large dielectric anisotropy and refractive index anisotropy have been synthesized until now, and some of them have been used for practical purposes. For example, a four-ring compound having a bonding group CF2O is disclosed in the patent documents No. 1 to No. 6. However, since these compounds do not have sufficiently high clearing points, the temperature range of a display device containing the composition that includes the compound is not sufficiently wide.
The patent documents No. 7 to No. 10 discloses five-ring compounds having a tetrahydropyran ring and a bonding group CF2O [the compounds (S-1) to (S-3)]. The patent documents No. 11 and No. 12 disclose compounds having a tetrahydropyran ring and a dioxane ring [the compounds (S-4) and (S-5)]. However, these compounds do not have a sufficiently wide temperature range of a liquid crystal phase or a sufficiently large refractive index anisotropy and dielectric anisotropy.
